Work on the mechanism of control of the biosynthesis of methionine and other members of the aspartate family of amino acids has concentrated on in vivo studies to outline physiologically important regulatory patterns. (1) Lysine and threonine in combination inhibited growth of Lemna completely at concentrations which individually had little effect on growth. The inhibition was relieved by methionine, homoserine or homocystine. These observations are consistent with the combination of lysine and threonine exerting concerted feedback control of the synthesis of the aspartate family of amino acids as an early step (e.g. that catalyzed by aspartokinase) that is required for synthesis not only of lysine and threonine, but also of methionine. (2) Growth of Lemna with 35SO42-either in the presence or absence of supplementary methionine demonstrated that de novo synthesis of the sulfur moiety of methionine can be reduced to less than 25 percent of its normal rate by feedback control by methionine, and that the conversion of cysteine to cystathionine (catalyzed by cystathionine gamma-synthase) is the probable site of this control. (3) A potent inhibition of growth of lema by the aminoethoxy and methoxy derivatives of vinylglycine was demonstrated. Present data are consistent with derivatives acting primarily by inhibition of Beta cystathionase. The derivatives therefore promise to be valuable tools for specific inhibiton of the transsulfuration pathway.